Drilled pier foundations belong to the same category as pile foundations. No sharp deviations can be made between piers and piles because both of them serve similar purpose. The distinctions are based on the method of installation. A pile is installed by driving, whereas a pier is installed by excavating. Thus, a foundation unit installed in a drill hole may also be called a bored cast-in-situ concrete pile. Here, distinction is made between a small diameter pile and a large diameter pile. A pile, cast-in-situ, with a diameter less than 0.75 m (or 2.5 ft) is sometimes called a small diameter pile. A pile greater than the above size, is called a large diameter bored-cast-in-situ pile. The latter definition is used in most non-American countries whereas in the USA, such large diameter bored piles are called drilled piers, drilled shafts, and sometimes drilled caissons.
Different Types of Drilled Piers
Drilled piers may be described under four types. All four types are similar in construction technique, but differ in their design assumptions and in the mechanism of load transfer to the surrounding earth mass. These types are illustrated below.
1. Straight-shaft end-bearing piers
Straight-shaft end-bearing piers develop their support from end-bearing on strong soil, “hardpan” or rock. The overlying soil is assumed to contribute nothing to the support of the load imposed on the pier (Fig.-1).
2. Straight-shaft side wall friction piers
Straight-shaft side wall friction piers pass through overburden soils that are assumed to carry none of the load, and penetrate far enough into an assigned bearing stratum to develop design load capacity by side wall friction between the pier and bearing stratum (Fig.-2).
3. Combination of straight shaft side wall friction and end bearing piers
Combination of straight shaft side wall friction and end bearing piers are of the same construction as the two mentioned above, but with both side wall friction and end bearing assigned a role in carrying the design load. When carried into rock, this pier may be referred to as a socketed pier or a “drilled pier with rock socket” (Fig.-3).
4. Belled or under reamed piers
Belled or under reamed piers are piers with a bottom bell or under ream (Fig.-4). A greater percentage of the imposed load on the pier top is assumed to be carried by the base.